Seek issue，use too much time.

```
private boolean isUseCache(GetRequest request) throws ProxyCacheException {
long sourceLength = source.length();
boolean sourceLengthKnown = sourceLength > 0;
long cacheAvailable = cache.available();
// do not use cache for partial requests which too far from available cache. It seems user seek video.
// return !sourceLengthKnown || !request.partial || request.rangeOffset <= cacheAvailable + sourceLength * NO_CACHE_BARRIER;
return !sourceLengthKnown || !request.partial || request.rangeOffset <= cacheAvailable;// + sourceLength * NO_CACHE_BARRIER;
}
```

HTTP MP4 file, if the duration too long, "sourceLength * NO_CACHE_BARRIER" well be a **BIG number**, fetch cache from network will coast too much time, seek blocked.
Remove the added number, seek works normally.